It is well known that in a normal joint cartilage, a good balance is maintained between synthesis and destruction of the cartilage matrix, while in a pathological joint cartilage, destruction of the cartilage matrix has been accelerated irrespective of the type of the lesion. Degradation products of the cartilage matrix are first released into synovial fluid, then distributed into blood, and metabolized. Thus the amount and quality of each component of the cartilage matrix released into synovial fluid is an important source of information on the metabolism of the matrix. Proteoglycan, the main component of the cartilage matrix, is characterized by its being capable of forming an aggregate with hyaluronan to build up the huge mass of matrix structure together with the type II collagen in the cartilage matrix. The amount and quality of aggrecan, the main proteoglycan in the cartilage matrix, and its sugar chain, glycosaminoglycan, is a clue to know whether the destruction of the cartilage matrix has been accelerated.
The conventional method for assay of normal aggrecan consists in measurement of the amount of the hyaluronan-aggrecan aggregate isolated by ultracentrifugation (Heinegard, D., et al., J. Biol. Chem.,249, 4250 (1974)) and gel chromatography (Iwata, H., Shin Seikagaku Jikken Koza, Vol.3, Carbohydrates II, p.452-455, Apr. 15, 1991, published by Tokyo Kagaku Dojin Kabushiki Kaisha). The method is time-consuming and requires not a small amount of specimen.
Therefore an improved method for assay of normal aggrecan is now required; namely a method that requires a shorter time and a smaller amount of specimen. Also a method has been desired that can clarify the quantitative relationship between the change in normal aggrecan and the pathological state of a joint to judge whether the joint is pathological or normal and identify the type of the pathological joint, as the basis of diagnosis of arthritis or decision of a therapeutical plan.
As described above, proteoglycan in synovial fluid is useful as a marker molecule for diagnosis of arthritis, though in some types of arthritis the concentration of proteoglycan is lower than that in the normal joint; thus the increased concentration of proteoglycan does not always indicate that the joint is pathological.